The transmission of digital communication signals, such as digitized voice or data, over a time division multiplexed (TDM) serial communications system, such as a T1telecommunications network used to transport digitized telephone signals at a bit rate of 1.544 Mb/s, customarily involves the assembly of successive telephone channels into frames of multiple channels of digital data. For example, a typical DS1 data frame contains twenty-four (64 Kb/s DSO) TDM telephone signalling channels, having eight bits per channel (plus a framing bit, for a total of 193 bits per frame). The frames are assembled into superframes (of twelve frames each), and transmitted over the serial TDM link.
Most telecommunication carriers also offer fractional T1 circuits, in increments of 56 Kb/s and 64 Kb/s, which allow a user to employ less than a full T1 circuit between two points in the network. Connection is made to the same network elements, with multiple users sharing the same interoffice T1 bandwidth. In order to interface a T1 network site with customer data terminal equipment (DTE), such as a router, computer, local area network bridges, computer aided design systems, teleconferencing equipment and the like, a service provider may install a piece of data communication equipment (DCE), for example, a T1-rate CSU/DSU (customer service unit/data service unit), such as a TSU, manufactured by Adtran Corp., Huntsville, Ala., having a single Nx56 Kbps/64 Kbps data port. The bandwidth of the data port is determined by the number of DSO channels that have been purchased from the T1 service provider, the DTE may occupy contiguous or alternate channels in the T1 serial time slot stream, and the respective channels may start at any position.
In order to strobe or clock data sourced from the DTE to the DCE, with the data then being asserted by the DCE onto the T1 link, the user typically may choose among a variety of clock sources, including the DTE clock, and normal and inverted versions of a transmission strobe clock signal selectively available at the DCE, either externally or internally during sampling of the data. The availability of these complementary versions of the DCE transmission strobe clock signal allows the user to manually select the best clock phase that will compensate for propagation delays, such as those resulting from long DTE signalling cables, and internal driver and receiver circuits of the DTE.
Unfortunately, even though the user has the ability to select different transmission strobe clock signals, an inherent problem with such a user selection technique is the fact that the parameters of each piece of data communications equipment are vendor specific, which confuses the customer, who must conduct a trial and error process, that inevitably results in the customer calling the equipment supplier to resolve the problem. This initial confusion problem is compounded by the fact that the DTE side of the DCE may also undergo a configuration change that causes a change in propagation delay, and may necessitate a change in the choice of transmission strobe clock signal.